Insulating Unvented Roof Assemblies: Foam Free

We’re big fans of Fine Homebuilding. In the most recent issue of Fine Homebuilding, Martin Holladay wrote a piece on Insulating Unvented Roof Assemblies. We agree that avoiding sheathing rot is critical, and that air permeable insulation such as fiberglass or cellulose cannot be used alone. But in it, unfortunately, all four options provided are dominated by foam. They did not consider fibrous insulation options installed with the addition of an interior air barrier, sans foam. Nor did Fine Homebuilding consider an alternative to foam boards above the sheathing – where wood fiberboard like GUTEX or mineral wool boards would be credible and preferred options. And the Options 3 and 4 provided are potential disasters to homeowners – where toxic spray foam is applied to the underside of an existing roof of a too often occupied building. Like we’re stuck in a dystopian 1960’s sci-fi movie, it’s still just foam, foam, foam.

There is a more sustainable and robust way. 475 believes that foam fails, and should be avoided. Less is Best is our motto. Therefore, in addition to simply replacing foam boards with GUTEX or Roxul, we have proposed some alternative ways to insulate an unvented roof that integrate the use of an interior air barrier, INTELLO PLUS, which is also a smart vapor retarder and airtight membrane – providing a previously unachievable degree of protection from moisture damage.

The membrane changes permeability from .17 perms in the winter to over 13 perms in the summer, depending on the relative humidity around it, allowing the insulation to dry out faster and more reliably.

One of the main reasons for the use of foam is its vapor retarding properties. Using INTELLO PLUS with thermal insulation provides the necessary air and vapor barrier needed to prevent structural damage.

As we note in our blog post Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass – A WUFI Post, the building code, written for the foam industry, does ask for air-impermeable insulation, or foam. However, the intent of the code can be readily met with an assembly that includes fibrous insulation and an inboard air barrier and vapor control layer – and is regularly approved for use as such. If you are in a cold climate and are considering one of our suggested assemblies, review it with your local building department official. We are happy to help you.

Here are our own 4 options for building an unvented roof assembly, all including INTELLO PLUS and all foam free.

1. Fiberglass Batts
Fiberglass is an inexpensive insulative material, coming in at an average of 1/3 – 1/4 the price of spray foam. Fiberglass batts allow for easy installation and can be purchased at higher densities to meet your insulation needs. Fiberglass is composed of spun glass fibers, making it naturally flame retardant, and can be made from recycled materials. The main drawback of batt insulation is the potential for gaps in the insulative layer if it is not correctly cut to fill the entire cavity.

2. Mineral Wool
Like fiberglass, mineral wool can be installed in batts. Since it is made from non-directional stone fibers, the insulation is water repellent, microbial growth resistant, fire resistant, and denser than other insulative materials. It also provides superior sound control. Mineral wool has an R-value comparable to spray cellulose or high density fiberglass batts.

3. Dense-packed Cellulose
Cellulose is a good choice for the especially environmentally conscious, as it is made from recycled materials. The added fire retardant is borate, the same thing you find the your all-natural cleaners and laundry detergents. It is non-toxic, affordable and provides excellent thermal control. Intello Plus is especially well suited for dense pack cellulose, as it is reinforced and can withstand the pressure from the insulating material.

4. Blown-in Fiberglass
Blown fiberglass may be chosen over batt fiberglass as it provides more complete coverage. Where batts have the potential to leave spaces when installed, blown insulation comes in smaller pieces and fills those potential gaps.

All of these installations perform best with an airtight, vapor-open weather resistant barrier. SOLITEX MENTO 1000 is a popular choice, as it is weather resistant, vapor open and allows the insulating layer to dry out. For extra reinforcement suitable for dense-pack insulation, choose SOLITEX MENTO PLUS. You’ll find examples of all of the above in real world projects on our video index. These projects are excellent examples of how to achieve foam free and high performance on a budget you can afford.

We have details that speak to that assembly. If you download our ebooks (https://foursevenfive.com/product-category/quality-control/books/) you’ll find foam free floor assembly details in the Ground Connections section of each. Essentially you can treat the floor as another wall, with exterior membrane on the underside (Solitex Mento Plus can support dense pack), joists with cellulose and your taped plywood inboard of the insulation. Let us know if that works for you. We’re happy to help further.

In the concluding paragraph, you say “these installations perform best with an exterior vapor barrier/weather resistant barrier”. Was that meant to be “an exterior AIR barrier/weather resistant barrier”?

in Sacramento,CA we do not have an extremely cold climate, but there are those periods of cold that make there way through, and then in California we are required to install fire suppression, so within that insulated roof assembly is a fire piping system, the problem I have is making it air tight with all those sprinkler penetrations. then I have the potential of moisture forming at the underside of the water barrier on top of the roof sheathing. then with asphalt roofing shingles its all trapped, and rot is imminent .

You mentioned Borate is the fire retardant in cellulose, but omitted the use of ammonium sulphate by some cellulose manufacterers. Although it’s considered safe for humans, some brands outgas ammonia more than others, not a good thing for chemically sensitive people.

Great point. We talk about cellulose insulation so often, we sometimes forget to mention this, as it’s become second nature among many people we work with. We always recommending using cellulose that is free of ammonium sulphate. Just as we recommend investigating the binder and fire retardant used in fiberglass, or looking into the MDI content of your sheathing boards – not all brands are created equal.

One other thing, ammonium sulphate is acidic and can corrode metal, so it’s an important consideration for applicationswhere the insulation will come in contact with fasteners, etc. Another reason to use the Borate-only version.

I did closed cell on my living room unvented ceiling but have to more unvented ceilings left to do and wonder exactly what you are doing here. I used Roxul R23 in my walls and wanted to use the R30 in my ceilings but was unaware of any solution outside of foam board on top of roof sheeting or closed cell on under side of sheeting. You are saying I can do it another way? I built this house as a builder who likes testing methods and wanted to try all kinds of theories in my own home. So if you could explain to me the exact method for using Roxul in my ceilings that are unvented Im all ears! Thanks

How do I know If have a unvented attic. I’m selling my house and the buyer asked to put vents on the roof because there was none. Now I see there are unvented roofs. In my attic , looking down towards the eves there is a clear looking sheet that goes around my whole attic. do you know what that means and what its for ?

We can’t be certain about the current state of your situation without seeing it. If you email us with photos we may be able to help more.

But it sounds like you may have a vapor barrier up there, and that the buyer wants vents installed as a way to release the condensation build-up in the attic. To create an unvented assembly, you need an air barrier that doesn’t trap the moisture. By replacing vapor barriers with vapor-smart membranes, you allow moisture to dry without losing efficiency.

Thanks as always for the great posts. We’ve got a large home in High Park in Toronto that we are currently renovating and we have a large sloped roof to insulate. The challenge is that we’ve got the typical rafter embedded in the exterior masonry walls scenario (masonry completely fills the rafter spaces at the exterior walls) so getting ventilation into the rafter cavities would need to be done with something like Cor-a-vent which gets installed above the ice dam level (3′ or so depending on climate) on the shingle side. The trick is that this clients roof is new and lining each and every internal rafter cavity with baffles (we use thermopan for this as it’s easy to install) is very time consuming. SO, I love the idea of a unventilated assembly but I want to make absolutely sure we aren’t going to present any moisture issues over time.

We have details that speak to that assembly. If you download our ebooks (https://foursevenfive.com/product-category/quality-control/books/) you’ll find foam free floor assembly details in the Ground Connections section of each. Essentially you can treat the floor as another wall, with exterior membrane on the underside (Solitex Mento Plus can support dense pack), joists with cellulose and your taped plywood inboard of the insulation. Let us know if that works for you. We’re happy to help further.

In the concluding paragraph, you say “these installations perform best with an exterior vapor barrier/weather resistant barrier”. Was that meant to be “an exterior AIR barrier/weather resistant barrier”?

in Sacramento,CA we do not have an extremely cold climate, but there are those periods of cold that make there way through, and then in California we are required to install fire suppression, so within that insulated roof assembly is a fire piping system, the problem I have is making it air tight with all those sprinkler penetrations. then I have the potential of moisture forming at the underside of the water barrier on top of the roof sheathing. then with asphalt roofing shingles its all trapped, and rot is imminent .

You mentioned Borate is the fire retardant in cellulose, but omitted the use of ammonium sulphate by some cellulose manufacterers. Although it’s considered safe for humans, some brands outgas ammonia more than others, not a good thing for chemically sensitive people.

Great point. We talk about cellulose insulation so often, we sometimes forget to mention this, as it’s become second nature among many people we work with. We always recommending using cellulose that is free of ammonium sulphate. Just as we recommend investigating the binder and fire retardant used in fiberglass, or looking into the MDI content of your sheathing boards – not all brands are created equal.

One other thing, ammonium sulphate is acidic and can corrode metal, so it’s an important consideration for applicationswhere the insulation will come in contact with fasteners, etc. Another reason to use the Borate-only version.

I did closed cell on my living room unvented ceiling but have to more unvented ceilings left to do and wonder exactly what you are doing here. I used Roxul R23 in my walls and wanted to use the R30 in my ceilings but was unaware of any solution outside of foam board on top of roof sheeting or closed cell on under side of sheeting. You are saying I can do it another way? I built this house as a builder who likes testing methods and wanted to try all kinds of theories in my own home. So if you could explain to me the exact method for using Roxul in my ceilings that are unvented Im all ears! Thanks

How do I know If have a unvented attic. I’m selling my house and the buyer asked to put vents on the roof because there was none. Now I see there are unvented roofs. In my attic , looking down towards the eves there is a clear looking sheet that goes around my whole attic. do you know what that means and what its for ?

We can’t be certain about the current state of your situation without seeing it. If you email us with photos we may be able to help more.

But it sounds like you may have a vapor barrier up there, and that the buyer wants vents installed as a way to release the condensation build-up in the attic. To create an unvented assembly, you need an air barrier that doesn’t trap the moisture. By replacing vapor barriers with vapor-smart membranes, you allow moisture to dry without losing efficiency.

Thanks as always for the great posts. We’ve got a large home in High Park in Toronto that we are currently renovating and we have a large sloped roof to insulate. The challenge is that we’ve got the typical rafter embedded in the exterior masonry walls scenario (masonry completely fills the rafter spaces at the exterior walls) so getting ventilation into the rafter cavities would need to be done with something like Cor-a-vent which gets installed above the ice dam level (3′ or so depending on climate) on the shingle side. The trick is that this clients roof is new and lining each and every internal rafter cavity with baffles (we use thermopan for this as it’s easy to install) is very time consuming. SO, I love the idea of a unventilated assembly but I want to make absolutely sure we aren’t going to present any moisture issues over time.

We have details that speak to that assembly. If you download our ebooks (https://foursevenfive.com/product-category/quality-control/books/) you’ll find foam free floor assembly details in the Ground Connections section of each. Essentially you can treat the floor as another wall, with exterior membrane on the underside (Solitex Mento Plus can support dense pack), joists with cellulose and your taped plywood inboard of the insulation. Let us know if that works for you. We’re happy to help further.

In the concluding paragraph, you say “these installations perform best with an exterior vapor barrier/weather resistant barrier”. Was that meant to be “an exterior AIR barrier/weather resistant barrier”?

in Sacramento,CA we do not have an extremely cold climate, but there are those periods of cold that make there way through, and then in California we are required to install fire suppression, so within that insulated roof assembly is a fire piping system, the problem I have is making it air tight with all those sprinkler penetrations. then I have the potential of moisture forming at the underside of the water barrier on top of the roof sheathing. then with asphalt roofing shingles its all trapped, and rot is imminent .

You mentioned Borate is the fire retardant in cellulose, but omitted the use of ammonium sulphate by some cellulose manufacterers. Although it’s considered safe for humans, some brands outgas ammonia more than others, not a good thing for chemically sensitive people.

Great point. We talk about cellulose insulation so often, we sometimes forget to mention this, as it’s become second nature among many people we work with. We always recommending using cellulose that is free of ammonium sulphate. Just as we recommend investigating the binder and fire retardant used in fiberglass, or looking into the MDI content of your sheathing boards – not all brands are created equal.

One other thing, ammonium sulphate is acidic and can corrode metal, so it’s an important consideration for applicationswhere the insulation will come in contact with fasteners, etc. Another reason to use the Borate-only version.

I did closed cell on my living room unvented ceiling but have to more unvented ceilings left to do and wonder exactly what you are doing here. I used Roxul R23 in my walls and wanted to use the R30 in my ceilings but was unaware of any solution outside of foam board on top of roof sheeting or closed cell on under side of sheeting. You are saying I can do it another way? I built this house as a builder who likes testing methods and wanted to try all kinds of theories in my own home. So if you could explain to me the exact method for using Roxul in my ceilings that are unvented Im all ears! Thanks

How do I know If have a unvented attic. I’m selling my house and the buyer asked to put vents on the roof because there was none. Now I see there are unvented roofs. In my attic , looking down towards the eves there is a clear looking sheet that goes around my whole attic. do you know what that means and what its for ?

We can’t be certain about the current state of your situation without seeing it. If you email us with photos we may be able to help more.

But it sounds like you may have a vapor barrier up there, and that the buyer wants vents installed as a way to release the condensation build-up in the attic. To create an unvented assembly, you need an air barrier that doesn’t trap the moisture. By replacing vapor barriers with vapor-smart membranes, you allow moisture to dry without losing efficiency.

Thanks as always for the great posts. We’ve got a large home in High Park in Toronto that we are currently renovating and we have a large sloped roof to insulate. The challenge is that we’ve got the typical rafter embedded in the exterior masonry walls scenario (masonry completely fills the rafter spaces at the exterior walls) so getting ventilation into the rafter cavities would need to be done with something like Cor-a-vent which gets installed above the ice dam level (3′ or so depending on climate) on the shingle side. The trick is that this clients roof is new and lining each and every internal rafter cavity with baffles (we use thermopan for this as it’s easy to install) is very time consuming. SO, I love the idea of a unventilated assembly but I want to make absolutely sure we aren’t going to present any moisture issues over time.